I do remember Emily creating a virus accidentally during one of her experiments but thought that it had been eradicated in those affected by and orgy in Dr Jessica's surgery and destroying what remained of it physically in the lab. As I remember it Emily's virus was neither airborne nor transmissible, by bodily fluids, as usual with sexually transmitted diseases, which is why only women who had swallowed that version of the serum ended up presenting symptoms. (Namely Tasha, Jessica, Alia, Kaley and Pricia.) Since the only possible vector of spread for the Garner virus was imbibing her serum directly, i.e., orally or possibly inhaling aerosol, how could the citizens of North Santiva become infected by such a virus unknowingly when to contract it they have to swallow an nasty tasting liquid or had it sprayed into their faces and inhaled?
That doesn't make any sense to me but then this is a NLT Media game we're talking about here, which normally have next to no sense or reason in them and where basically anything goes.
That said viruses do mutate randomly and so maybe Emily Garner's creation changed. Or perhaps a former resident of Cape Vedra carried the virus to North Santiva, knowingly or unknowingly, which puts Henry Johnson, Tasha Cole, Diana Dimitriadis, Duncan Helmer, Sister Judy and Madalyn Rose in the frame as the only characters that have visited and/or lived in both areas as far as we know.
And looking at the list of names one above all others suggests deviltry, mischief and malevolence to me!